The use of a cache to store temporary copies of data items retrieved from memory allows the latency associated with retrieving data items from memory to be reduced and furthermore to reduce the energy expenditure associated with retrieval of those data items. However, a cache itself has a leakage power cost to the system in which it is implemented and in a large cache this power expenditure can be considerable. Steps may be taken to reduce the leakage of a cache, when that cache is provided in a number of portions (e.g. banks), for example by powering down one or more portions of the cache when it is determined that a smaller cache will adequately meet the processing requirements of the system. Hence the benefit of the reduced leakage power resulting from powering down a portion of the cache can be gained. Nevertheless determining when it is appropriate to power down a portion of the cache is not easy and the opportunity remains to improve such cache power management decision making processes.